While their exact nature is unknown, due to its natural loot location from Forsworn Briarhearts, it seems they really do serve as actual hearts; from the look of events at Lost Valley Redoubt, dead Forsworn are resurrected in a ceremony by hagravens, replacing their human hearts with these items. Interestingly enough, pickpocketing the briar heart from a Forsworn Briarheart will instantly kill the target.

The base magnitude of the Fortify Block effect for this ingredient is 2 instead of 4. This is only relevant if honeycomb and briar heart are combined, in which case the resulting potion will have a 50% smaller magnitude than a typical custom Fortify Block potion.

Events during the Repairing the Phial quest can make all Briar Hearts quest items permanently, meaning you can't drop them or use them in any way. See the quest page for tips on how to avoid this, or fix it if it has already occurred.